1. Field of the Invention
The present invention relates to an endless belt which can be used in a press apparatus such as an Extended Nip Press or Intensa S Press or the like for dewatering a wet web of paper in a paper making process.
2. Description of the Background Art
Recently, a so-called shoe press wherein, in the press part of a paper making process, dewatering of a web is carried out by pressing one surface of a web placed on a felt for enhancing the effect of dewatering the web with a rotary roll and pressurizing the other surface through an endless belt with a pressure shoe is becoming popular.
FIGS. 12 is a typical cross sectional view illustrating an Extended Nip Press as an example of such a press. Referring to FIG. 12, a pressure shoe 41 is arranged under a rotary roll 40. An endless belt 42 as a dewatering press is provided between rotary roll 40 and pressure shoe 41. Endless belt 42 is mounted and wound around guide rolls 43a and 43b and tension rolls 44a, 44b and 44c. A web 45 and felt 46 are passed between the endless belt 42 and the rotary roll 40.
If the rotary roll 40 is rotated in a direction indicated by arrow A, web 45 placed on felt 46 and endless belt 42 are moved in directions indicated by arrow B and arrow C, respectively. Web 45 and felt 46 are pressed strongly toward the rotary roll 40 by the pressure shoe 41 in the range of the pressure dewatering part P. Accordingly, web 45 is dewatered in pressure dewatering part P.
FIG. 13 is a typical cross sectional view illustrating an Intensa S Press as another example. Referring to FIG. 13, a cylindrical endless belt 52 for a dewatering press is provided under a rotary roll 50. A pressure shoe 51 pressing toward the rotary roll 50 through endless belt 52 is provided inside the endless belt 52. A web 54 and a felt 53 are passed between endless belt 52 and rotary roll 50. A pressure dewatering part P having a large width is formed between rotary roll 50 and pressure shoe 51. Web 54 is dewatered in pressure dewatering part P.
The following are endless belts for dewatering presses conventionally proposed for such shoe presses.
(1) a belt in which a synthetic resin such as polyurethane resin or rubber is impregnated into a base fabric of an endless belt from one of its surfaces (hereinafter referred to as a single coat type belt: U.K. Patent No. 2, 106,555, U.K. Patent No. 2,106,557 and so forth) PA1 (2) a belt in which an endless base fabric is embedded in a layer of synthetic resin or rubber (hereinafter referred to as a base fabric embedded belt: European Patent No. 194,602 and so forth) PA1 (3) a belt in which a synthetic resin such as polyurethane resin or rubber is impregnated into an endless base fabric from both of its front and rear surfaces, and grooves are formed on the surface (hereinafter referred to as a double coat type grooved belt: U.S. Pat. Nos. 4,559,258, 4,908,103, 4,946,731 and so forth) PA1 (a) Since all of the single coat type belt, the base fabric embedded belt, and the double coat type grooved belt use an endless base fabric impregnated with a synthetic resin or a rubber, the strength of the bond between the base fabric and the synthetic resin or rubber is small in such belts. PA1 (b) An endless base fabric is normally formed of monofilaments such as polyamide fiber, polyester fiber, or the like. Such fiber is generally a material having a hardness higher than the hardness of the synthetic resin or rubber. Therefore, when the endless base fabric is bent during the conveying of the belt, the endless base fabric tends to be subject to a concentration of stress, and a breaking away phenomenon tends to occur between the base fabric and the synthetic resin or the rubber. The life of the conventional belt is also made relatively short for this reason. PA1 (c) Since the double coat type grooved belt is provided with grooves on its surface, moisture generated by dewatering of a web is held in the grooves. Therefore, the double coat type grooved belt has superior dewatering efficiency as compared with the single coat type belt. However, the double coat type grooved belt is formed on both its front and rear surfaces by the casting method or the like using a synthetic resin or a rubber. When one surface is formed after formation of the other surface, the texture of the base fabric is already tight, so that bubbles in the coating material do not disappear easily, and the belt is formed with the bubbles remaining. Accordingly, in the case of the double coat type grooved belt, such bubbles tend to be left in the resin or the rubber. If such a belt is used, white water which is pressurized during the pressurization of a rotary roll permeates from the bottom part of the grooves into the bubbles and further permeates from the bubbles into the base fabric. As a result, the breaking away phenomenon between the base fabric and a coating layer occurs at a relatively early time which shorten the life of the belt. PA1 (d) In addition, the conventional endless belts use endless base fabric. The tensile force of filaments in endless base fabric is often not uniform, and it often happens that the base fabric is distorted in the belt during the manufacturing process thereof. Therefore, the entire belt tends to receive the distortion and is correspondingly deformed, or wrinkles or the like tend to be generated.
However, the above-described conventional endless belts for dewatering presses have problems as follows.
Particularly, in the case where layers disposed on both the front and rear surfaces of the double coat type grooved belt are formed by the casting method, synthetic resin or rubber penetrates the base fabric from only one of the surfaces. Accordingly, it is not possible to cause the synthetic resin or rubber to penetrate the base fabric when the opposite surface is formed, so that a uniform anchoring effect from both sides cannot be obtained, and thus it is not possible to obtain a strong bond between the respective components. Therefore, there is the problem that, as the belt is used, a breaking away phenomenon is caused between the base fabric and the synthetic resin or the rubber which greatly reduces the life of the belt.